The invention relates to a vehicle transmission shift control system, and more particularly, to a clutch slip-based powershift transmission shift control system.
Modern agricultural powershift transmissions, such as the AG250 powershift transmission manufactured by Funk Manufacturing, contain multiple electronically controlled, hydraulically actuated wet clutches which direct the flow of power through the transmission. These clutches are grouped according to the portion of the transmission gear train section they control. The group of clutches which are connected directly to or through constantly meshing gears to the transmission output shaft are referred to as range clutches. Clutches between the transmission input and the range clutches are known as speed clutches. During shifting from one transmission input to output speed ratio to another speed ratio, the speed clutches change the flow of power through the transmission from one range clutch to another, resulting in an interim transmission ratio. This prepares the input section of the gear train for the final target transmission speed ratio. All clutch exchanges are controlled by fixed timing values based on a measured engine load. The load signal itself, however, cannot distinguish between the amount of engine power passing through the transmission and the amount absorbed by an external application, such as a power-take-off (PTO) application. It would therefore be desirable to sense only the transmission load, which would allow for precisely matched clutch engagements. When the transmission reaches the interim ratio, which is higher than the target ratio, the transmission output speed and vehicle ground speed will increase or decrease unless the torque transmitted by the off going range clutch is maintained at a constant transmission output torque. Therefore, it would be desirable for the range clutch to maintain its original static torque level in a new dynamic slip condition. Static coefficients of friction are typically different (usually larger) than dynamic coefficients of friction, and clutch torque capacity is dependent upon the coefficient of friction between the clutch discs. When the range clutch slips and transitions from static to dynamic operation, torque capacity changes, resulting in an abrupt shift which vehicle operators perceive as a bad shift.